If I format a Seagate ST3000DM001 with ext4, it gives 2145430320 K blocks. I'm used to a few % for overhead, but this looks like it's more like 30%. Normally on a 2TB Seagate I get 1949547144 K blocks. There's not much of a benefit in going to 3Tb disks at this rate. Anyone know the reason why this is?

Where do you read that from ?
If you use the dumpe2fs utility, you should be able to understand where your 30% have gone.
As well as what you can do to reduce this amount... a little.
If you still get problems, post the output of

Code:

dumpe2fs -h /dev/_your_sd_device

As well as your emerge --info

BTW : You did build your filesystem with auto_64-bit_support
Just asking this because your number of blocks is strangely close to the max of a signed 32 bits int._________________

" WARNING: The size of this disk is 3.0 TB (3000592982016 bytes).
DOS partition table format can not be used on drives for volumes
larger than (2199023255040 bytes) for 512-byte sectors. Use parted(1) and GUID
partition table format (GPT)."

But when it started with DOS partition table, I didn't pay any attention to it

Thanks everyone, I'll switch over to GPT. After so many years using fdisk it'll be a bit of a change for me.